Conventionally, for this type of oil pump, an oil pump is known that is disposed at a position spaced apart from a main shaft of an automatic transmission, and that is driven by power transmitted from a drive sprocket provided on a main shaft side to a driven sprocket attached to a rotation shaft of the oil pump by way of a chain (see e.g., Patent Document 1). In such oil pump, the rotation shaft of the oil pump is spline fitted to a boss portion of the driven sprocket, and a ball bearing is fixed to an outer circumference of the rotation shaft. The ball bearing fixed to the rotation shaft is held by a cylindrical portion formed on a pump cover that covers one end of a pump body, and a snap ring that prevents the ball bearing from falling out of the pump cover is interposed between an outer circumferential portion of the ball bearing and the cylindrical portion of the pump cover. The driven sprocket is thereby supported in a radial direction by the pump cover by way of the ball bearing, and the axial movement thereof with respect to the pump cover is regulated by the snap ring.